PROCESSOR 6502 INCLUDE "VCS.H" INCLUDE "SKELRAM.ASM" SEG ROM ORG $1000 DC.B "Skeleton (c)2002 Eric Ball",0 KERN0 STA WSYNC ;69-0 LDA (MAZE0L),Y ;-5 STA PF0 ;-8 STX PF1 ;-11 STX PF2 ;-14 LDX #5 ;-16 WAIT0 DEX ;+2 BNE WAIT0 ;+2/3 LDA (MAZE0R),Y ;40-45 STA PF0 ;-48 INY ;-50 LDA WALL ;-53 CMP #1 ;-55 BEQ WALL15 ;-57/58 CPY #20 ;-59 BNE KERN0 ;-61/62 BEQ KERN20 ;-64 WALL15 CPY #15 ;58-60 BCC KERN0 ;-62/63 DEX ;-64 CPY #20 ;-66 BNE KERN0 ;-68/69 WALL20 STA WSYNC ;44-0 LDA #$80 ;-2 STA PF0 ;-5 LDA #$00 ;-7 STA PF1 ;-10 STA PF2 ;-13 LDX #170-20 ;-15 JSR XWALL ;-21 DEX ;36-38 JMP KERN170 ;-41 KERN20 STA WSYNC ;72-0 TXA ;-2 AND #$0F ;-4 ORA (MAZE1L),Y ;-9 STA PF1 ;-12 STX PF2 ;-15 LDX #$80 ;-17 STX PF0 ;-20 LDX #4 ;-22 WAIT20 DEX ;+2 BNE WAIT20 ;+2/3 AND #$0F ;41-43 ORA (MAZE1R),Y ;-48 STA PF1 ;-51 INY ;-53 LDA WALL ;-56 CMP #2 ;-58 BEQ WALL35 ;-60/61 CPY #40 ;-62 BNE KERN20 ;-64/65 BEQ KERN40 ;-67 WALL35 CPY #35 ;61-63 BCC KERN20 ;-65/66 DEX ;-67 CPY #40 ;-69 BNE KERN20 ;-71/72 WALL40 STA WSYNC ;41-0 LDA #$10 ;-2 STA PF1 ;-5 LDA #$00 ;-7 STA PF2 ;-10 LDX #150-40 ;-12 JSR XWALL ;-18 DEX ;36-38 JMP KERN150 ;-41 KERN40 STA WSYNC ;72-0 LDA (MAZE2L),Y ;-5 STA PF1 ;-8 STX PF2 ;-11 LDX #5 ;-13 CPY SKELTOP ;-15 BCC WAIT40 ;-17/18 LDA (SKEL0),Y ;-22 STA GRP0 ;-24 LDA (SKEL1),Y ;-29 STA GRP1 ;-32 LDX #2 ;-34 WAIT40 DEX ;+2 BNE WAIT40 ;+2/3 LDA (MAZE2R),Y ;42/43-47/48 STA PF1 ;-50/51 INY ;-53 LDA WALL ;-56 CMP #3 ;-58 BEQ WALL55 ;-60/61 CPY #60 ;-62 BNE KERN40 ;-64/65 LDA #$00 ;-66 BEQ KERN60 ;-69 WALL55 CPY #55 ;61-63 BCC KERN40 ;-65/66 DEX ;-67 CPY #60 ;-69 BNE KERN40 ;-71/72 WALL60 STA WSYNC ;41-0 LDA #$11 ;-2 STA PF1 ;-5 LDA #$00 ;-7 STA PF2 ;-10 LDX #130-60 ;-12 JSR XWALL ;-18 DEX ;36-38 JMP KERN130 ;-41 PVIEW4 LDA $00,X ; right/left step 2 subroutine BEQ PVIEW5 ; open found, check next LDA #>TYPE0 ; closed, show wall BNE PVIEW7 PVIEW5 LDA $01,X ; is next open too? BEQ PVIEW6 LDA #>TYPE1 ; open, show hole BNE PVIEW7 PVIEW6 LDA #>TYPE2 ; both open, show Y PVIEW7 INX ; next test position STA $0001,Y ; pointer MSB to RAM LDA #$00 STA $0000,Y ; LSB = 0 INY ; next pointer position INY RTS WAIT60 LDX #3 ;20-22 WAIT61 DEX ;+2 BNE WAIT61 ;+2/3 BEQ KERN61 ;36-39 KERN60 STA WSYNC ;70-0 AND #$F0 ;-2 ORA (MAZE3L),Y ;-7 STA PF2 ;-10 LDX #$11 ;-12 STX PF1 ;-15 CPY SKELTOP ;-17 BCC WAIT60 ;-19/20 TAX ;-21 LDA (SKEL0),Y ;-26 STA GRP0 ;-29 LDA (SKEL1),Y ;-34 STA GRP1 ;-37 TXA ;-39 KERN61 AND #$F0 ;39-41 ORA (MAZE3R),Y ;-46 STA PF2 ;-49 INY ;-51 LDA WALL ;-54 CMP #4 ;-56 BEQ WALL75 ;-58/59 CPY #80 ;-60 BNE KERN60 ;-62/63 BEQ KERN80 ;-65 WALL75 CPY #75 ;59-61 BCC KERN60 ;-63/64 LDA #$FF ;-65 CPY #80 ;-67 BNE KERN60 ;-69/70 KERN80 STA WSYNC ;36-0 LDA #$08 ;-2 STA PF2 ;-5 LDX #110-80 ;-7 JSR XWALL ;-13 DEX ;36-38 KERN110 STA WSYNC ;56-0 LDA (MAZE3L),Y ;-5 STA PF2 ;-8 LDX #4 ;-10 CPY SKELBOT ;-12 BCS WAIT110 ;-14/15 LDA (SKEL0),Y ;-19 STA GRP0 ;-22 LDA (SKEL1),Y ;-27 STA GRP1 ;-30 LDX #1 ;-32 WAIT110 DEX ;+2 BPL WAIT110 ;+2/3 LDA (MAZE3R),Y ;39/41-44/46 STA PF2 ;-47/49 INY ;-51 CPY #130 ;-53 BNE KERN110 ;-55/56 KERN130 STA WSYNC ;64-0 LDA (MAZE2L),Y ;-5 STA PF1 ;-8 STX PF2 ;-10 LDX #4 ;-12 CPY SKELBOT ;-14 BCS WAIT130 ;-16/17 LDA (SKEL0),Y ;-21 STA GRP0 ;-24 LDA (SKEL1),Y ;-29 STA GRP1 ;-32 LDX #1 ;-34 WAIT130 DEX ;+2 BPL WAIT130 ;+2/3 LDA (MAZE2R),Y ;41/43-46/48 STA PF1 ;-49/51 INY ;-53 CPY #135 ;-55 BCC KERN130 ;-57/58 LDX #$00 ;-59 CPY #150 ;-61 BNE KERN130 ;-63/64 LDX #$FF ;-65 KERN150 STA WSYNC ;64-0 LDA (MAZE1L),Y ;-5 STA PF1 ;-8 STX PF2 ;-10 LDX #4 ;-12 CPY SKELBOT ;-14 BCS WAIT150 ;-16/17 LDA (SKEL0),Y ;-21 STA GRP0 ;-24 LDA (SKEL1),Y ;-29 STA GRP1 ;-32 LDX #1 ;-34 WAIT150 DEX ;+2 BPL WAIT150 ;+2/3 LDA (MAZE1R),Y ;41/43-46/48 STA PF1 ;-49/51 INY ;-53 CPY #155 ;-55 BCC KERN150 ;-57/58 LDX #$00 ;-59 CPY #170 ;-61 BNE KERN150 ;-63/64 LDX #$FF ;-65 KERN170 STA WSYNC ;64-0 LDA (MAZE0L),Y ;-5 STA PF0 ;-8 STX PF1 ;-10 STX PF2 ;-12 LDX #5 ;-14 WAIT170 DEX ;+2 BPL WAIT170 ;+2/3 LDA (MAZE0R),Y ;43-48 STA PF0 ;-51 INY ;-53 CPY #175 ;-55 BCC KERN170 ;-57/58 LDX #$00 ;-59 CPY #190 ;-61 BNE KERN170 ;-63/64 LAST LDA #30*76/64+1 STA WSYNC ;-0 STA TIM64T ;-4 STX PF0 LDX SOUNDEL ; sound countdown BEQ COLUPA ; already zero? DEX BNE COLUPA ; zero? STX AUDV0 ; turn off sound STX AUDV1 COLUPA STX SOUNDEL LDA SKELIFE ; skeleton still alive? BPL COLUPB LDX FIREDEL LDA SKELDIE,X BNE COLUPC BEQ COLUPC COLUPB LDA FIREDEL ; fire flash? CMP #55 BCS FIRETST LDA MAZECOL ; set playfield colour STA COLUPF LDX SKELIFE ; set skeleton color LDA LIFELUM,X COLUPC STA COLUP0 STA COLUP1 FIRETST LDA INPT4 ; fire button AND #$80 CMP THISFIR ; debounce BNE PLAYMOV ; not the same LDX LASTFIR ; check last button position STA LASTFIR ; save current BPL PLAYMOV ; do nothing if last was down ASL ; check current BCS PLAYMOV ; do nothing if up LDA FIREDEL ; check reload delay BNE PLAYMOV ; do nothing if reloading STA AUDF0 ; BANG! STA AUDF1 LDA #8 STA AUDC0 STA AUDC1 STA AUDV0 STA AUDV1 STA SOUNDEL LDA SKELDIS ; is skeleton in view? AND #$03 BEQ SETDEL ; not in view LDA #$00 ; make skeleton turn SEC ; and face plaer SBC PLAYDIR STA SKELNXT LDA RANDOM ; generate random damage AND #$0F ; 0-15 STA FIREDEL ; save temp LDA SKELIFE ; reduce skeleton life SEC SBC FIREDEL STA SKELIFE LDA FIREDEL ; set skeleton colour flash ASL ASL ASL ASL STA FIREDEL ; save msn LDX SKELIFE ; set luminance LDA LIFELUM,X ORA FIREDEL ; combine STA COLUP0 STA COLUP1 SETDEL LDA MAZECOL ; set playfield colour AND #$0F ; flash maze too STA COLUPF LDA #60 ; set reload timer = 1 sec STA FIREDEL BNE PLAYMOV PLAYMOV LDA SWCHA ; PLAYER MOVE ORA #$0F CMP THISMOV ; DEBOUNCE BNE SKELMOV LDX LASTMOV ; moved last frame? CPX #$FF BNE PMOVEA ; no CMP #$EF ; forward? BEQ PMOVEC CMP #$7F ; turn right? BEQ PMOVEB CMP #$BF ; turn left? BNE SKELMOV STA LASTMOV LDX PLAYDIR ; turn left LDA #$00 SEC SBC PLAYRIG STX PLAYRIG STA PLAYDIR BNE SKELMOV PMOVEA CMP #$FF ; return to center? BNE SKELMOV STA LASTMOV ; centered BEQ SKELMOV PMOVEB STA LASTMOV ; right turn LDX PLAYRIG LDA #$00 SEC SBC PLAYDIR STX PLAYDIR STA PLAYRIG BNE SKELMOV PMOVEC STA LASTMOV ; step forward LDA WALL CMP #1 BEQ SKELMOV LDA PLAYPOS CLC ADC PLAYDIR STA PLAYPOS CMP SKELPOS BNE SKELMOV JMP GOTYOU SKELMOV LDA SKELIFE ; skeleton still alive? BMI SKELSTP DEC SKELDEL ; main skeleton movement delay BEQ SKDEL1 SKELSTP JMP BOTTOM ; no movement, skip everything SKDEL1 LDA SKELSEC ; decrease rate counter SEC SBC SKELPER STA SKELSEC BCS SKDEL2 ; no carry = underflow ADC #60 STA SKELSEC LDA SKELMIN SEC SBC SKELPER ; decrease square counter STA SKELMIN BCS SKDEL2 ; no carry = underflow ADC #87 STA SKELMIN LDA SKELHRS SEC SBC SKELPER ; decrease cube counter STA SKELHRS BCS SKDEL2 ; no carry = underflow ADC #76 STA SKELHRS DEC SKELPER ; speed up skeleton! SKDEL2 LDA SKELPER STA SKELDEL LDA SKELNXT ; forced move? BEQ SMOVEA ; nope, start the decision tree LDX #0 ; clear forced move flag STX SKELNXT CMP SKELDIR ; forced=facing? BNE SKNXT0 JMP SMOVE1 ; step forward SKNXT0 STA SKELDIR ; turn and set SKELRIG correctly CMP #1 BEQ SKNXT1 BMI SKNXT2 LDA #-1 STA SKELRIG BNE SKELSTP SKNXT1 LDA #16 STA SKELRIG BNE SKELSTP SKNXT2 CMP #-1 BEQ SKNXT3 LDA #1 STA SKELRIG BNE SKELSTP SKNXT3 LDA #-16 STA SKELRIG BNE SKELSTP SMOVEA LDX #0 ; direction flags LDA SKELPOS ; can we go forward? CLC ADC SKELDIR TAY LDA LVLMASK AND MAZES,Y BNE SMOVED ; can't go forward INX ; forward flag LDA #$01 ; determine facing BIT SKELDIR ; use bit to test for neg too BNE SMOVEB ; SKELDIR = +1/-1 LDA PLAYPOS ; check for player in same column SEC SBC SKELPOS AND #$0F BNE SMOVED JMP SMOVE1 SMOVEB BMI SMOVEC ; SKELDIR = -1 LDA PLAYPOS SEC SBC SKELPOS AND #$F0 ; player ahead BNE SMOVED JMP SMOVE1 SMOVEC LDA SKELPOS ; player ahead (behind) SEC SBC PLAYPOS AND #$F0 ; player ahead BNE SMOVED BEQ SMOVE1 SMOVED LDA SKELPOS ; can we go right? CLC ADC SKELRIG TAY LDA LVLMASK AND MAZES,Y BNE SMOVEE INX INX SMOVEE LDA SKELPOS ; can we go left? SEC SBC SKELRIG TAY LDA LVLMASK AND MAZES,Y BNE SMOVEF INX INX INX INX SMOVEF CPX #0 BEQ SMOVE0 ; no directions, turn around DEX BEQ SMOVE1 ; forward only DEX BEQ SMOVE2 ; right only DEX BEQ SMOVE3 ; right or forward DEX BEQ SMOVE4 ; left only DEX BEQ SMOVE5 ; left or forward DEX BEQ SMOVE6 ; right or left BIT RANDOM ; all directions BPL SMOVE1 BMI SMOVE6 SMOVE0 LDA #$00 ; turn around SEC SBC SKELDIR STA SKELDIR LDA #$00 SEC SBC SKELRIG STA SKELRIG JMP BOTTOM SMOVE2 LDX SKELRIG ; turn right LDA #$00 SEC SBC SKELDIR STX SKELDIR STA SKELRIG STX SKELNXT ; force forward JMP BOTTOM SMOVE3 BIT RANDOM BPL SMOVE1 BMI SMOVE2 SMOVE4 LDX SKELDIR ; turn left LDA #$00 SEC SBC SKELRIG STX SKELRIG STA SKELDIR STA SKELNXT ; force forward JMP BOTTOM SMOVE5 BIT RANDOM BPL SMOVE1 BMI SMOVE4 SMOVE6 BIT RANDOM BVC SMOVE2 BVS SMOVE4 SMOVE1 LDA SKELPOS ; step forward CLC ADC SKELDIR STA SKELPOS CMP PLAYPOS BNE SMOVE7 JMP GOTYOU SMOVE7 LDA SOUNDEL ; check for existing sound BNE BOTTOM JSR SKPOSXY ; calculate distance for sound JSR SKSTEP LDA #1 STA AUDC0 STA AUDC1 LDA RANDOM ORA #30 STA AUDF0 STA AUDF1 CPX #-15 BPL AUDX LDX #-15 AUDX STX AUDV0 ; left speaker STX COLUP0 CPY #-15 BPL AUDY LDY #-15 AUDY STY AUDV1 ; right speaker ; STY COLUP1 LDA SKELDEL LSR STA SOUNDEL BOTTOM JSR WAITIM LDA #$02 STA VSYNC JSR RAND ; cycle LFSR LDA #$01 STA CTRLPF ; reflect playfield STA WSYNC ; set initial skeleton position LDX #8 ;-2 SKPOS DEX ;+2 BNE SKPOS ;+2/3 STA RESP0 ;46-49 LDA #$60 ; shift left half left STA RESP1 STA HMP0 LDA #$40 ; center right half (final position) STA HMP1 STA WSYNC STA HMOVE LDA #38*76/64+1 STA WSYNC STA TIM64T LDA #$00 STA VSYNC STA HMCLR NEWSKEL LDA SKELIFE ; skeleton dead? BPL PVIEW LDA FIREDEL ; done death flash? BNE PVIEW LDX SKELNUM ; increment skeleton life INX STX SKELNUM STX SKELIFE LDX MAZECOL ; increment maze luminance INX TXA AND #$0F CMP #13 BEQ NEXTLVL ; ten down, new level STX MAZECOL LDA #$00 STA SKELPOS JSR SKPOSXY ; figure out restart position ; LDA SKELX BPL NEWSK1 EOR #$FF ; NEG CLC ADC #$01 NEWSK1 BIT SKELY BPL NEWSK2 SEC SBC SKELY BPL NEWSK3 NEWSK2 CLC ADC SKELY NEWSK3 CMP #$07 BPL NEWSK4 LDA #120 STA SKELPOS NEWSK4 LDX #LOADDEL-LOADRAM; reset skeleton move timers BNE REINIT NEXTLVL TXA ; increment maze colour CLC ADC #$16 STA MAZECOL LDX #LOADLVL-LOADRAM ASL LVLMASK BNE REINIT JMP YOUWIN REINIT LDA LOADRAM-1,X ; refresh initial values STA RAMLOAD-1,X DEX BNE REINIT PVIEW LDX #0 ; center view & skeleton distance STX SKELDIS LDY PLAYPOS JSR PVIEW1 BNE PVIEWA JSR PVIEW1 BNE PVIEWA JSR PVIEW1 BNE PVIEWA JSR PVIEW1 BNE PVIEWA INX PVIEWA STX WALL ; save wall distance LDA PLAYPOS ; right view, step 1 CLC ADC PLAYRIG LDX #MAZE1R+1 ; save bits here PVIEWE JSR PVIEW3 ; test and save bits CPX #MAZE3R+2 BNE PVIEWE LDA PLAYPOS ; left view, step 1 SEC ; SBC PLAYRIG ; LDX #MAZE1L+1 ; save bits here PVIEWF JSR PVIEW3 ; CPX #MAZE3L+2 ; BNE PVIEWF ; LDX #MAZE1R+1 ; right view, step 2 LDY #MAZE0R ; PVIEWG JSR PVIEW4 ; change bits to pointers CPY #MAZE3R+2 ; BNE PVIEWG ; LDX #MAZE1L+1 ; left view, step 2 LDY #MAZE0L ; PVIEWH JSR PVIEW4 ; change bits to pointers CPY #MAZE3L+2 ; BNE PVIEWH ; LDA SKELDIS ; skeleton view AND #$03 ; check distance BNE SKPOSA STA SKELBOT ; force skeleton off SBC #1 ; by reversing top & bottom STA SKELTOP JMP SKVIEW SKPOSA SEC ; decrement for table access SBC #1 ASL ; multiply by 16 ASL ASL ASL TAY LDA SKTABLE,Y ; use table to update RAM STA HMP0 LDA SKTABLE+1,Y STA SKELTOP LDA SKTABLE+2,Y STA SKELBOT LDA SKTABLE+3,Y STA NUSIZ0 STA NUSIZ1 LDA PLAYDIR ; figure out facing CMP SKELDIR BEQ SKPOSB ; same dir = back CMP SKELRIG ; left BEQ SKPOSC CLC ADC SKELDIR BEQ SKPOSD ; inverse = front LDA SKTABLE+8,Y ; must be right STA SKEL1 ; arm on the right LDA SKTABLE+9,Y STA SKEL1+1 LDA SKTABLE+10,Y ; body on the left STA SKEL0 LDA SKTABLE+11,Y STA SKEL0+1 LDA #$18 ; mirror both BNE SKVIEW SKPOSB LDA SKTABLE+6,Y ; back for both STA SKEL0 STA SKEL1 LDA SKTABLE+7,Y STA SKEL0+1 STA SKEL1+1 LDA #$08 ; left normal, right mirror BNE SKVIEW SKPOSC LDA SKTABLE+8,Y ; arm on the left STA SKEL0 LDA SKTABLE+9,Y STA SKEL0+1 LDA SKTABLE+10,Y ; body on the right STA SKEL1 LDA SKTABLE+11,Y STA SKEL1+1 LDA #$00 ; no mirror BEQ SKVIEW SKPOSD LDA SKTABLE+4,Y ; front for both STA SKEL0 STA SKEL1 LDA SKTABLE+5,Y STA SKEL0+1 STA SKEL1+1 LDA #$08 ; left normal, right mirror SKVIEW STA WSYNC ; hmove P0 for size STA HMOVE STA REFP1 ; set reflection registers LSR STA REFP0 STA WSYNC STA HMOVE JSR WAITIM LDA INPT4 ; save button for debounce AND #$80 STA THISFIR LDX FIREDEL ; decrement reload counter BEQ RELOAD DEX STX FIREDEL RELOAD LDA SWCHA ; save joystick for debounce ORA #$0F STA THISMOV LDY #$00 LDX #$00 JMP KERN0 PVIEW1 INX ; center view subroutine TYA ; move forward one step CLC ADC PLAYDIR CMP SKELPOS ; check skeleton position BNE PVIEW2 STX SKELDIS ; save if found PVIEW2 TAY ; check for wall LDA LVLMASK AND MAZES,Y ; BNE done after return RTS PVIEW3 TAY ; right/left step 1 subroutine LDA LVLMASK ; move bits from ROM to RAM AND MAZES,Y STA $00,X INX TYA CLC ; move forward one step ADC PLAYDIR RTS SKPOSXY LDA SKELPOS ; change SKPOS-PLAYPOS to SKELX+SKELY SEC SBC PLAYPOS STA SKELX LSR LSR LSR LSR BIT TESTBIT ; negative? BEQ SKXY2 ORA #$F0 SKXY2 STA SKELY LDA SKELX BIT TESTBIT ; negative? BEQ SKXY3 INC SKELY ORA #$F0 BMI SKXY4 SKXY3 AND #$0F SKXY4 STA SKELX RTS SKSTEP LDA #$01 BIT PLAYDIR BEQ SKSTEPG ; PLAYDIR = 16 or -16 BMI SKSTEPD ; PLAYDIR = -1 LDA SKELX ; PLAYDIR = 1 BMI SKSTEPA ; skeleton behind EOR #$FF ; NEG CLC ADC #$01 BMI SKSTEPB SKSTEPA ASL ; multiply by 2 SKSTEPB STA SKELX LDA SKELY BPL SKSTEPC ; skeleton right CLC ADC SKELX TAX ; left = X + Y CLC ADC SKELY TAY ; right = X + 2Y RTS SKSTEPC LDA SKELX SEC SBC SKELY TAY ; right = X - Y SEC SBC SKELY TAX ; left = X - 2Y RTS SKSTEPD LDA SKELX ; PLAYDIR = -1 BMI SKSTEPE ; skeleton ahead EOR #$FF ; NEG CLC ADC #$01 ASL ; multiply by 2 STA SKELX SKSTEPE LDA SKELY BPL SKSTEPF ; skeleton left CLC ADC SKELX TAY ; right = X + Y CLC ADC SKELY TAX ; left = X + 2Y RTS SKSTEPF LDA SKELX SEC SBC SKELY TAX ; left = X - Y SEC SBC SKELY TAY ; right = X - 2Y RTS SKSTEPG BMI SKSTEPK ; PLAYDIR = -16 LDA SKELY ; PLAYDIR = 16 BMI SKSTEPH ; skeleton behind EOR #$FF ; NEG CLC ADC #$01 BMI SKSTEPI SKSTEPH ASL ; multiply by 2 SKSTEPI STA SKELY LDA SKELX BPL SKSTEPJ ; skeleton left CLC ADC SKELY TAY ; right = Y + X CLC ADC SKELX TAX ; left = Y + 2X RTS SKSTEPJ LDA SKELY SEC SBC SKELX TAX ; left = Y - X SEC SBC SKELX TAY ; right = Y - 2X RTS SKSTEPK LDA SKELY ; PLAYDIR = -16 BMI SKSTEPL ; skeleton ahead EOR #$FF ; NEG CLC ADC #$01 ASL ; multiply by 2 STA SKELY SKSTEPL LDA SKELX BPL SKSTEPM ; skeleton left CLC ADC SKELY TAX ; left = Y + X CLC ADC SKELX TAY ; right = Y + 2X RTS SKSTEPM LDA SKELY SEC SBC SKELX TAY ; right = Y - X SEC SBC SKELX TAX ; left = Y - 2X RTS ; simple 8 bit LFSR RAND LDA RANDOM ;+3 BEQ XSEED ;+5/6 LSR ;+7 BCC SRAND ;+9/10 XSEED EOR #$A9 ;+6/9-8/11 SRAND STA RANDOM ;+8/10/11-11/14 RTS ;+17/20 WAITIM STA WSYNC ;-0 LDA INTIM ;-4 BNE WAITIM ; wait for bottom of frame RTS XWALL0 STA WSYNC ;-0 XWALL CPY SKELTOP ;-2 BCC XWALL1 ;-4/5 CPY SKELBOT ;-6 BCS XWALL1 ;-8/9 LDA (SKEL0),Y ;-13 STA GRP0 ;-16 LDA (SKEL1),Y ;-21 STA GRP1 ;-24 XWALL1 INY ;-26 DEX ;-28 BNE XWALL0 ;-30/31 RTS ;-36 START SEI ; enable interupts CLD ; turn off BCD math LDA #$00 ; clear zero page memory & registers LDX #$44 ; start with the TIA shadow NULL STA 0,X INX BNE NULL DEX ; set stack to bottom of RAM TXS STX COLUPF ; set up title screen LDX #